This disclosure is related to methods of directed self-assembly, and the layered structures formed therefrom; and more specifically, to self-assembly of microdomains of block copolymers having higher spatial frequency.
The ability to pattern features with smaller critical dimensions allows denser circuitry to be fabricated, thereby enabling more circuit elements within the same area and reducing the overall cost per element. Features having smaller critical dimensions and tighter pitch are needed in each technology generation. Directed self-assembly (DSA) of polymeric materials is a potential candidate to extend current lithography by enhancing the spatial resolution and/or controlling the critical dimension variation of a predefined pattern on a substrate. In particular, DSA of block copolymer (BCP) materials and polymer blends have been explored for this purpose. There are two major approaches to the directed self-assembly of BCP thin films: graphoepitaxy and chemical epitaxy.
In the graphoepitaxy method depicted in Prior Art FIG. 1A, the self-organization of block copolymers is guided by topographical features of pre-patterned substrates. A topographically patterned substrate with sidewalls 910 having a preferential 1 affinity for one domain 914 and an underlying orientation control layer 912 can be used to direct self-assembly (of lamellar domains in this case) within the trench of width L. For example, in FIG. 1A self-aligned lamellar domains 914 and 916 of BCPs form parallel line-space patterns in topographical trenches and enhance pattern resolution by subdividing the space (trench) of the lithographically-derived topographical pre-pattern. One domain, for example domain 916, can be selectively etched leaving domain 914. With a trench of width L and BCP with a periodicity of PBCP, frequency multiplication of a factor of L/PBCP can be achieved. However, graphoepitaxy has issues which potentially limit its utility as a lithographic technique. For example, in graphoepitaxy, the placement error and line-edge roughness of BCP domains are deteriorated by the imperfect topographical pre-patterns and non-uniform thickness of the BCP polymer layer coated over the topographical substrate. In addition, the mandrels 918 used to direct the self-assembly process are usually much larger than the BCP domains, and the area occupied by the mandrels is essentially wasted. Also, many graphoepitaxy approaches use topographic features formed by etching into an oxide hard mask layer to provide patterns that are stable to the BCP casting solvent. The additional process steps using the hard mask layer increase the cost and complexity of graphoepitaxy-based DSA processes.
Alternatively, in the chemical epitaxy method depicted in Prior Art FIG. 1B, the self-assembly of BCP materials to form domains 926 and 928 is directed by dense chemical patterns in a layer 920. The pitch of the chemical pattern (PS) is roughly equivalent to the pitch of the BCP domain periodicity (PBCP). The preferential affinity between at least one of the chemical pattern regions 922 or 924 and a corresponding BCP domain 926 or 928 directs the self-assembly of the BCP domains in accordance with the underlying chemical pre-pattern. Unlike graphoepitaxy, no area is lost to the features directing self-assembly. In addition, the BCP self-assembly process improves the dimensional control due to the lower critical dimension variation (both mean critical dimension and line-edge roughness (LER) or line-width roughness (LWR)) in the final self-assembled structures compared to that in the underlying chemical pre-pattern. Even though DSA using chemical epitaxy on dense chemical patterns increases critical dimension control, it does not improve resolution or provide an information gain. Current optical lithography tools do not have sufficient resolution to print these 1:1 chemical patterns. Instead, these patterns have been fabricated using lithographic techniques such as e-beam direct write or extreme-ultraviolet (EUV, 13.5 nm) interference lithography which are not suitable for volume manufacturing.
Chemical epitaxy on sparse chemical patterns, however, can provide critical dimension and orientation control similar to that on dense chemical patterns but also provide enhanced resolution. For example, Prior Art FIG. 1C shows the directed self-assembly of BCP material to form domains 936 and 938 on a sparse chemical pattern layer 930 comprised of alternating pinning regions 934 having a width WP=0.5*PBCP and regions with an orientation control material 932 having a width WCA=PS−WP In the example shown in FIG. 1C, pinning regions 934 have a strong affinity for domains 938. An orientation control material such as 932 having operationally equivalent affinity for both domains 938 and domains 936 is suitable to support perpendicularly-oriented domains (936 and 938) as shown. The factor of frequency multiplication is determined by the ratio of the pitch of the sparse chemical patterns stripe (PS) and the pitch of BCP (PBCP). In FIG. 1C, the depicted ratio PS/PBCP=2, which would result in frequency doubling. Similarly, spatial frequency tripling could be achieved by using a sparse chemical pattern having a PS/PBCP=3, and so on.
While a wide variety of orientation control materials have been developed including surface-bound self-assembled monolayers, polymer brushes (i.e., the polymer chains attach at one end to an interface and extend away from the interface to form a “brush” layer), and photo- or thermally-crosslinkable polymer materials, in practice it is quite difficult to fabricate suitable sparse chemical patterns that effectively direct self-assembly of BCPs in a manner that can be integrated with conventional lithographic processes and materials. Prior Art FIGS. 2A to 2D show some of the approaches that have been used. FIG. 2A illustrates direct writing of 1:1 dense chemical patterns by e-beam direct write lithography or EUV interference lithography in an orientation control layer 940 (e.g., a surface-bound polymer brush or self-assembled monolayer). The chemical pattern generated in layer 940 directs self-assembly of a BCP to form domains 942 and 944. Although e-beam lithography can be used to make sparse chemical patterns in this manner, e-beam direct write lithography is not suitable for volume manufacturing. Alternatively, as shown in Prior Art FIG. 2B, a conventional positive-tone photoresist 946 can be patterned on top of an orientation control material 948 to form patterned photoresist features 950. The photoresist pattern can protect the underlying surface during an etch process that creates pinning regions (i.e., a region having a particular affinity for one domain of a self-assembled material), either by removing the material to uncover the underlying substrate or by inducing enough damage to the material that it becomes preferential to one of the BCP self-assembled domains 954 or 956. The protecting photoresist layer can then be removed by an organic solvent rinse leaving the etched orientation layer having features 952. However, the special orientation control material may interfere with the photoresist patterning by causing reflectivity or photoresist adhesion issues. In addition, this method is quite process intensive and involves etch processes that cannot be done in the lithography track tool. Furthermore, the protective photoresist layer may be damaged sufficiently by the etch process that it cannot be stripped cleanly and completely. Rather than selectively removing or altering an orientation control material, negative-tone photo-patternable orientation control materials 958 have been developed that can be patterned on top of conventional anti-reflective coatings 960 (Prior Art FIG. 2C). BCP self-assembled domains 962 and 964 are also shown in Prior Art FIG. 2C. In practice, while such negative-tone photo-patternable orientation control materials work well for large area patterns, no such materials have demonstrated sufficient resolution performance. Finally, as shown in Prior Art FIG. 2D, direct patterning of a crosslinking negative-tone photoresist 966 on top of an orientation control material 968 can produce pinning regions. BCP self-assembled domains 970 and 972 are also shown in Prior Art FIG. 2D. Care must be taken so as to not alter the surface properties of the orientation control material in the non-exposed regions during photoresist casting and development. In addition, it is challenging to achieve high resolution patterning using a dark field mask with the limited range of conventional negative-tone photoresists for optical lithography. The best reported efforts have used a hydrogen silsesquioxane negative-tone e-beam photoresist, which is unsuitable for volume manufacturing.
While patterning a thin positive-tone photoresist on a controlled surface seems to be the most straightforward approach to creating chemical patterns to direct self-assembly, in practice many problems are encountered that make this route unviable. For example, while patterning resists with typical thicknesses of 80 to 200 nm is suitable for the generation of topographical patterns required for graphoepitaxy, patterning high fidelity, low defect images in the thin (e.g., sub-15 nm) layers required for chemical patterns is difficult. In addition, the underlying surface must serve as both a bottom anti-reflective coating (BARC) during photoresist patterning and later as an orientation control layer in the areas cleared during patterning. However, commercially available BARCs (being optimized primarily for their optical properties, etch properties, and photoresist adhesion/profile performance) do not have the appropriate surface properties to act as orientation control materials. In addition, current orientation control materials do not have the thickness or optical properties necessary to provide adequate reflection control (especially for high numerical aperature (high-NA) optical lithography). In practice, it is quite difficult to maintain the surface properties of the underlying orientation control material through the course of conventional lithographic patterning of positive-tone photoresist. The underlying orientation control material in the exposed regions is typically subjected to the imaging radiation (e.g., damaging deep ultraviolet (DUV), extreme ultraviolet (EUV), or electron beam radiation), superacids at elevated temperatures (i.e., photo-generated acid during post-exposure bake), reactive intermediates (e.g., those from photoacid generator (PAG) decomposition, acidolysis of photoresist protecting groups, or photodecomposition of photoresist materials), and strong bases (i.e., aqueous tetramethylammonium hydroxide during photoresist development). Any and all of these can change the surface properties of the underlying orientation control material. And since the specifics and magnitude of the changes are dependent upon the lithographic exposure technique, the photoresist, and the processing conditions, it is not possible to compensate for all of these effects by manipulating the orientation control material.
Therefore, methods of generating sparse chemical patterns for directing self-assembly of a material are needed that are compatible with conventional tools, processes, and materials.